Je. Martin et E. Meiburg, THE GROWTH AND NONLINEAR EVOLUTION OF HELICAL PERTURBATIONS IN A SWIRLING JET MODEL, European journal of mechanics. B, Fluids, 17(4), 1998, pp. 639-651
The growth and nonlinear evolution of a helical perturbation is invest
igated in a simplified swirling jet model, consisting of a line vortex
along the axis surrounded by a jet shear layer with both azimuthal an
d streamwise vorticity. Inviscid Lagrangian vortex dynamics simulation
s demonstrate the mechanisms of vorticity concentration, reorientation
, and stretching, as well as the nonlinear interaction and competition
between a centrifugal Rayleigh instability and a Kelvin-Helmholtz ins
tability feeding on both components of the base flow vorticity. The no
nlinear evolution resulting from the interaction of these two instabil
ities allows for very different flow behaviors to emerge, depending on
whether the helical perturbation wave and the vortex lines of the jet
shear layer wind around the jet axis in the same or in opposite direc
tions. Large-scale vortex helices evolve that can contain azimuthal vo
rticity either of the same or of opposite sign to that initially prese
nt in the jet shear layer. These different evolutions are triggered by
the differences in the direction of the strain field set up by the ev
olving large-scale helix. In both cases, the generation of both signs
of azimuthal vorticity due to the centrifugal Rayleigh instability all
ows for the possibility of unlimited growth of the helix circulation,
in the absence of viscous diffusion. (C) Elsevier, Paris.